X-ray diffractometry (XRD) is a well-known technique for studying the crystalline structure of matter. In XRD, a sample is irradiated by a monochromatic X-ray beam, and the locations and intensities of the diffraction peaks are measured. The characteristic diffraction angles and the intensity of the diffracted radiation depend on the lattice planes of the sample under study and the atoms that occupy those planes. For a given wavelength λ and lattice plane spacing d, diffraction peaks will be observed when the X-ray beam is incident on a lattice plane at angles θ that satisfy the Bragg condition: nλ=2d sin θ, wherein n is the scattering order. The angle θ that satisfies the Bragg condition is known as the Bragg angle. Distortions in the lattice planes due to stress, solid solution, or other effects lead to observable changes in the XRD spectrum.
XRD has been used, inter alia, for measuring characteristics of epitaxial films produced on semiconductor wafers. For example, Bowen et al. describe a method for measuring germanium concentration in a SiGe structure using high-resolution XRD in “X-Ray metrology by Diffraction and Reflectivity,” Characterization and Metrology for ULSI Technology, 2000 International Conference (American Institute of Physics, 2001), which is incorporated herein by reference.
XRD may also be used at grazing incidence to observe structures on the surface of a sample. For example, Goorsky et al. describe the use of grazing-incidence XRD for analyzing epitaxial layer structures on a semiconductor wafer in “Grazing Incidence In-plane Diffraction Measurement of In-plane Mosaic with Microfocus X-ray Tubes,” Crystal Research and Technology 37:7 (2002), pages 645-653, which is incorporated herein by reference. The authors apply the technique to determine the in-plane lattice parameter and lattice orientation of very thin surface and buried semiconductor layers.